Aerosol compositions, including air treatment compositions, are known to be provided in metal cans, such as steel cans. Such compositions generally have a high volatile organic content (VOC) level. In recent years, it has become desirable to reduce in part or whole the VOC of aerosol compositions. As a result, the volatile solvents of the compositions have been replaced in part with water thereby generally giving the aerosol compositions a higher aqueous content. The aqueous content, however, serves to corrode the metal of the cans holding the compositions for storage and dispensing. Steel as used for constructing conventional aerosol cans is often pre-plated with a tin barrier coating in order to protect the underlying steel layer against atmospheric corrosion (rusting). The same tin-plating may also provide protection of the underlying steel layer against corrosion in aqueous environments. The interior of tin-plated steel cans may or may not have an organic coating (lining) or polymeric laminate in order to provide some additional protection of the tin and steel layers against corrosion. It has been found, however, that while tin plating, with or without a lining, initially protects against such corrosion, that the tin plating itself over time may dissolve in aqueous formulations thereby revealing the underlying steel layer. This dissolution of the tin plating into the aqueous formulation serves to degrade the fragrance present and to discolor the formulation. Formula discoloration is essentially a result of reactions between fragrance molecules in aqueous solution at an alkaline pH, i.e., a pH of greater than 7. When the pH is greater than 7, the aqueous system is alkaline/nucleophilic which is an electron-rich system seeking an electron-poor center. The hydroxide ions act either as base-leads to condensation reactions and/or as nucleophile-reactants with electron deficient carbonyl carbon of an ester resulting in ester hydrolysis. Fragrance molecules and ketones containing a carbonyl group, when treated with a base/nucleophile, forms an enolate anion resulting in unsaturated ketones and creates further conjugation in a molecule which absorbs longer wavelengths of UV/visible light producing undesirable discoloration. A steel can can also have traces of iron from either the container or from machining in making the container which can also create a variety of problems resulting in discoloration. Degradation of the fragrance and discoloration of the formulation both affect the functioning of the composition, such as the ability to freshen air with a fragrance of an air freshening composition and the ability to provide a formulation substantially free of discoloration. To combat this failing, combinations of corrosion inhibitors have been incorporated into the aqueous aerosol formulations so as to allow continued use of steel cans, tin-plated steel cans, or other cans subject to corrosion, rather than the more expensive aluminum cans.
One example of a compressed gas aerosol composition in a steel can is disclosed in WO 2011/138620 A1. The aerosol composition includes a borate salt as a corrosion inhibitor. Other corrosion inhibitors include nitrite salts and phosphate salts. Most corrosion inhibitors, however, due to their type of chemistry and the providing of the composition with a more highly alkaline pH (i.e., greater than 9 or 10), detrimentally affect any fragrance present in the composition, both hedonically and color-wise. Color is generally affected by a darkening of the formula and is generally undesirable, for example when the formula becomes darker than a pale yellow since this discoloration generally represents change in structure of the fragrance components which may adversely affect formula stability in terms of color, corrosion and hedonics. A compressed gas aerosol product may be visible on light or white surfaces or soft surfaces due to the larger particle size (fall out) and low evaporation rate. Thus, darkening of the formula may have an undesirable visible result.
Further, steel cans by their nature require a formulation contained therein to be in a certain pH range, generally a pH of 4 to 14, preferably pH of 6 to 12.5, and more preferably a pH of 7 to 9, to avoid general surface rusting and localized forms of corrosion like pitting and crevice corrosion of the steel surface and significant tin-plating dissolution. In view of this and since the pH affects the fragrance stability and coloration of the composition, the pH level also must be addressed.
The above limitations, therefore, greatly affect the fragrance pallet available for use, i.e., the number and types of fragrances compatible for use with the aqueous formulation in a steel can, as well as coloration, which in turn can affect shelf and storage life, as well as consumer acceptance for appropriate hedonics and use on soft surfaces. The invention addresses these disadvantages of the art.